38 TRANSACTIONS LIVERPOOL BIOLOGICAL SOCIETY. 
late years, and Ascidians are now regarded as the degener- 
ate descendants of a very lowly-developed group of the 
early Vertebrata (or more correctly, Chordata). 
But before we are in a position to understand this 
important matter, it is well to have some knowledge of 
the ascidian structure at several different stages in the 
life-history, and we shall commence with the last stage 
of all—the full-grown, or adult condition. 
MopE oF OCCURRENCE. 
Ascidians are all marine, and they have been found in 
all seas, from tropical to polar regions, and at depths 
varying from near high-water mark down to three and a 
quarter miles or so. Round most of our own coasts they 
are common, and some kinds are familiar enough, under 
the name of ‘ sea-squirts,’ to many sea-side visitors, as 
being amongst the inhabitants of rock-pools which can be 
collected and kept in aquaria. Such forms are usually | 
found as dome-shaped bodies of a dull red colour, adhering 
to the rock or sea-weed, and having two small openings on 
their upper surface from which, when touched, they emit 
delicate jets of sea-water with considerable foree—thus 
establishing their claim to the title of ‘‘ sea-squirts’’; and 
their resemblance to double-necked leather bottles, whence 
the name Ascidian (from the Greek acxés). 
Others again form flat gelatinous expansions attached 
to sea-weeds or stones, and symmetrically marked with 
bright spots of colour in the form of circles, meandering 
lines, or starlike patterns. These are really colonies in 
which each spot of colour or ray of the star represents an 
ascidiozooid or member of the colony equivalent to the 
whole animal in the case of the solitary Ascidian. 
By dredging around the coast, outside low-water mark, 
many other kinds of Ascidians are found, of diverse forms 
